Labels are defined as destination locations for the GOTO statement. The colon, followed by a label name defines a label:

Labels only are visible within the procedure where they are defined. As a consequence, you may define labels with the same name in different procedures. Within one procedure however, a label name may be used only once. Between the colon and the label name no whitespace is required.
Example
IF info.version = "2.0" GOTO v2stuff
PRINT "TFR " FMT "d08" SCALE 1000.0 tx.frequency
GOTO finished
:v2stuff
PRINT "TFR " FMT "f1.3" tx.frequency
:finished